Serr Drop cure produces dramatic results for walnuts

If it is so easy, why isn't everyone doing it? You can bet just about every Serr walnut producer in California tried this spring to roll in the dough. Even non-Serr producers are wanting in on the action, even if there is little chance for success.

At least half the Serr walnut acreage in the state was treated in 2006 with a plant growth regulator, ReTain from Valent, that veteran Kings and Tulare counties. University of California Cooperative Extension Bob Beede discovered three years ago has a dramatic affect on what growers know as “Serr drop.”

With growers reporting Serr nut set improvement of two and three times past sets after applying ReTain at early flowering and yield increases ranging from 1,200 to 1,500 pounds more walnuts per acres, Pest Control Advisers' (PCA) cell phones were ringing out of pickup phone cradles this spring with growers wanting to know when ReTain would be applied.

One grower last season, Beede said, pocketed an extra $110,000 in income from a 50-acre Serr orchard after he applied ReTain in two directions in 20 gallons of water at the rate of eight-tenths of a bag of product.

“He got 2,200 pounds more walnuts per acre. At $1 per pound, you do the math. He made enough in one year off 50 acres to buy his own sprayer,” said Beede.

The PRG is expensive; $250 to $300 per acre to apply. However, with returns like that, it is easy to understand why Serr growers were bugging PCAs to get the product on this spring.

Officially called Pistillate Flower Abscission (PFA), “Serr drop” has been a decades-old nightmare for walnut growers. There are about 20,000 acres of Serr planted in the state. It can be an excellent yielder, however, PFA can cause as many as 90 percent of the flowers to abort, relegating Serr the dog of the walnut varieties because all Serr orchards in California have PFA, according to Beede.

This is the scientific explanation for PFA: “Excessive pollination in some cultivars (primarily Serr) will cause autocatalytic ethylene production, leading to embryo abortion and flower abscission.” Beede discovered in a single limb test in 2003 that the active ingredient in ReTain, aminioethoxyvinylglycine (AVG for short) inhibits the ethylene biosynthesis pathway, thereby significantly reducing ethylene production in plant tissues.”

What all that means is that pollen, mostly from other, nearby walnut varieties overwhelms Serr flowers and they abort. If Serrs are interplanted with Chandler, Tehama, Hartley, Vina or Chico varieties or are adjacent to orchards planted to those varieties, Serr flowers will likely drop like snowflakes in a Minnesota blizzard.

Growers and researchers have been acutely aware of the cause of Serr drop for years. It can be mitigated by shaking down excessive catkins with a tree shaker.

“The problem with that is you cannot always get a 10-ton shaker into a wet orchard or get the pollen load down enough on a Chandler to solve the problem,” said Beede at a recent Valent-sponsored meeting for PCAs and growers in Visalia, Calif.

Beede explained that walnuts and almonds only need about 30 percent of the blooms they put out to set a normal crop. For example, walnuts set enough flowers for 800 pounds of nuts per tree. However, a tree can only mature about 150 to 175 pounds of nuts per tree.

As good as Beede's 06 trials and grower results were last season, there were total failures. In Central California, the failures were related to timing and possibly application. Beede said one large grower saw no response from the product, likely because it was applied too early. In Northern California results have not been as positive as Central California. It may be weather related there.

“ReTain is all about timing and execution,” said Beede, comparing it to the use of gibberellins in table grapes.

ReTain must be applied when five to 30 percent of pistillate flowers reach peak pollen receptivity.

“Seventy percent of walnut farmers do not know what they are looking at during this period,” added Beede, telling PCAs they must be the ones who decide when best to apply the product.

Valent has available extensive literature showing the exact stages of Serr flowering to apply the product.

“We are talking about timing that may be 30 percent bloom by Thursday and by Monday you are too late,” he said. “You have got to walk the orchards. You cannot stop along side an orchard and asses it. Reflective light on the edge of an orchard puts trees on the edges four to five days ahead of the interior of the orchard.

“ReTain is not systemic. It is not translaminar. You have to hit the target. It is all about execution,” he emphasized.

Beede said with ReTain, walnut growers are entering the PGR world table grape growers have long known. “Why do you think table grape growers are out all night with those wrap-around spray rigs spraying gib? It is all about timing and absorption.” It is the same with this new walnut PGR.

If it means treating at 4 a.m. rather than 9 a.m. costing $10 more per acre, Beede said do it. If timing is off and/or application poor, $250 to $300 per acres is an expensive lesson.

In nine Beede trials last year, Serr yields increased from 1,200 to 1,500 pounds per acre. Results were similar with properly timed ground as well as helicopter applications.

With yields like that, non-Serr producers want in on the action. However, in two trials last year on the popular Chandler variety, Beede got no yield increase.

“I do not know if every year will be like that with Chandler. Last was an atypical year with low chill and poor bloom overlap,” he said.

“You definitely must have PFA for ReTain to give results. It is not an aphrodisiac. If your orchard is producing three tons per acre, I am not sure you can make a four-ton orchard. I would be very cautious about using ReTain in that situation. I would doubt there is a significant PFA problem and would question that there would be a long term benefit from using the PGR,” he added.